In a holographic universe model, could our 3D universe be encoded in 3D and still be a holographic universe, instead of 3D information encoded in 2D space (D information encoded in D-1 space)? Or that would make simply our universe (that according to the standard model, non an holographic one)?

The number of mathematical points in any dimensional universe is the same, so in principle yes. However the idea behind the holographic is to map the universe onto event horizons, which are assumed to be 2D surfaces. Not that anyone has a working theory right now.

A hologram is a three-dimensional image formed by the interference of light beams or other coherent light source. The interference interaction of the two connected and coherent light sources, create an image of 3-D. This is loosely similar to using our two eyes to create a three dimensional representation in space. In both cases, it is not really 3-D. You cannot touch a hologram with your hand and feel it be real 3-D. It is an illusion of 3-D, that can fool the eyes. It can appear to be 3-D, but to only one sense. it is not verified by a second sense.

The hologram is an affect I like to call a spatial illusion; 2.5-D. It only works in parts of the brain, but not the entire brain is fooled. Below is a 3-D illusion of a ball. This 2.5-D image may fool the eyes in thinking it is 3-D, but if you touch the computer screen, the illusion will go away.

If you used the same parts of the brain; tactic processing centers, but without actually touching, you would sense this is flat. The 2.5-D image is the limit of logic. Beyond 2.5-D, is what I call intuitive relief drawings. Like a relief drawing there is a partial 3-D aspect, but not full 3-D. Theory will get increasingly esoteric as we go beyond 2.5-D. You need to feel it in your guts; tactic assessment.

In terms of data processing within the brain, the flat or 2-D nature of the hologram; partial or left brain, is not connected to the x and y axis of 2-D space, but it really connected to the x and y axis of cause and affect. In the case of the holograph, the split laser beam, so the light is coherent, but slightly out phase, is a cause and affect that is split, such as by time, to create a phase difference, before they are made to overlap into the 2..5-D hologram.

Real 3-D theory is done differently. It uses three placement axis; cause, affect, cause. It will also be done as effect, cause and affect. In the case of touching a 3-D illusion, such as the red ball, to show it is flat, a secondary cause and affect associated with touch, unrelated to sight, is being used. It is not the same source of cause and effect as the eyes. But it does add more than cause and affect to the eyes. Such theory development has a different approach that can approach 3-D.

One may ask how would a 3-D approach to theory work in practical terms? This approach tries to define a phenomena in terms of two different sets of cause and affect, where both sets may not always overlap, but they need to lead to the same place. Each provides the extra or 3-D dimension in affect, cause and affect, and cause, affect, cause. For example, if we use seeing and touching at the same time, each has its own rules, until they merge. Luckily the right brain is set up to do this, directly. It is an acquired skill.

My best working example of this is connected to the biology of life. Life is a partnership between water and organics. You need both for life to operate. Theoretically, you can approach life from either the water or the organic side. Currently, we draw 2.5-D theory images from mostly the organic side. Not all these jive, with an approach from the water side; touch. But in the same token, not all observations jives with just the water side. You need to go in both directions, almost like two separate theories, being juggled at the same time, and merged until 3-D appears.

In physics we have the particle and the wave duality. We have wave theories and we have particle theories for the universe. These often stay apart, instead of work together as two paths to the same place. Or universal expansion and gravitational contraction are treated as two separate things, instead of merged as one connected phenomena. These are kept separate because it is not easy to make a 2.5-D hologram with two separate lasers. One laser and one mirror assembly is much easier.

The number of mathematical points in any dimensional universe is the same, so in principle yes. However the idea behind the holographic is to map the universe onto event horizons, which are assumed to be 2D surfaces. Not that anyone has a working theory right now.